The importance of effective and reliable endocardial leads in modern pacing systems is well recognized. Advances in the design of implantable pacemaker units have been spectacular, resulting in an array of long-life pulse generators which can provide pacing in modes that can be selected for the patient, and re-programmed to adapt to changes in patient conditions. But the pacing system needs to interface with the patient's heart, both to deliver the generated stimulus pulses and to sense spontaneous cardiac signals. It is the pacing lead that provides this interface, and its reliability and performance must match that of the pulse generator in order to ensure overall system functionality over the lifetime of the pacemaker.
In delivering stimulus pulses from the pacemaker unit to the patient's heart, the lead must perform the functions of carrying the pulses the length of the lead, and then delivering them to the heart wall, i.e., the myocardial tissue. This must be done in a manner such that the resulting electric field evokes depolarization of the cardiac cells and thus produces a cardiac contraction, either ventricular or atrial. The first task, that of carrying the pulses, requires integrity of the conductor (or conductors, for a bipolar lead) as well as the insulation that surrounds the conductor. The second task is complex. For a unipoloar or bipolar lead, an electrode is positioned at the distal tip, to provide an optimal electrode-myocardial interface. This interface must provide a good electrical connection, but in order to achieve this, a good mechanical connection must be provided in order that the electrode is well fixed, or anchored, with respect to the myocardium so as to provide stable chronic delivery of the stimulus pulses. Many anchoring mechanisms have been developed for employment at the distal end of endocardial leads, probably the most successful of which is the use of tines which engage the traebaculae and thus hold the distal tip in place. However, a further advance was provided by the development of the porous metal coated electrode. Such electrodes have a porous, or micro-porous surface which, when nestled against the heart wall, allows fibrous tissue ingrowth, resulting in stable long term fixation. Also, and importantly, the fibrous growth into the porous surface provides anchoring which reduces mechanical movement. In turn, this reduces the growth of connective tissue around the electrode surface which occurs as a reaction to the foreign body electrode, thus resulting in a reduced connective tissue "capsule" between the electrode surface and the excitable myocardial muscle. This is of great importance, as the capsule is not excitable, and acts as an effective extension of the distance between the electrode surface and the myocardial cells, which adversely affects both pacing threshold and sensing.
The importance of the porous surface is illustrated by considering the electric field strength established by a pulse of voltage V which is delivered to the electrode surface. For a spherical electrode, the field strength is inversely proportional to the square of (r+d), where r is the radius of the spherical electrode and d is the distance from the electrode surface to the boundary of the fibrous capsule and the undamaged myocardium. Thus, for a stimulus pulse of a voltage V, the field strength is reduced significantly by capsule thickness; stating the matter in an alternate way, the reduction of the capsule thickness by a porous surface provides a substantial improvement in pacing threshold. Capsule formation has been further reduced by the use of steroid elution, whereby a material which suppresses the myocardial/electrode interface reaction is eluted through the electrode surface, resulting in a lessened threshold rise as the electrode becomes fixed with respect to the myocardium.
Another important characteristic of the electrode is that of electrode impedance, i.e., the ratio of pulse voltage to the current drain at the electrode surface. Pacing impedance is inversely proportional to electrode surface area. Since it is desirable to provide a moderately high pacing impedance to reduce current drain while providing good power transfer, electrode surface should be as small as possible, although in practice there is an optimum surface size related to the thickness of the connective tissue capsule. Stokes et al developed the "Nano Tip" lead, which combines deep porosity, steroid elution, and a low surface area down to about 1.5 square mm. Stokes K, Bird T, "A New Efficient Nano Tip Lead", PACE, 1990; 13 (Part II): 1901-1905. This lead provides low pacing thresholds, and a pacing impedance in the range of about 1 to 1.2 Kohm, compared to an impedance of about 600 ohm for conventional electrodes of about 6-8 square mm surface. Additionally, Stokes showed that such small electrodes can provide good sensing, because of the high input impedance relative to the sensing impedance of the porous surface electrode. See Stokes K., "Do small electrodes have good sensing?", Reblampa. 1995; 8: 198-200. For good sensing, the source impedance should be small, so as to avoid attenuation when the sensed signal is connected to the input of a sense amplifier, which desirably has a high input impedance. While a larger electrode surface in principle provides a better (smaller) source impedance than a smaller surface, more porous surfaces also provide smaller source impedance; microporous surfaces provide even better sensing than porous surfaces.
However, the small surface electrode does not provide as much porous surface for tissue ingrowth as the larger surface, and this can present stability problems. Specifically, for a porous surface which is much smaller than the prior standard of 6-8 mm, there are indications that the stability of the electrode fixation may not be enough to guarantee clinical safety. Stable fixation is a key to maintaining a thin capsule and chronic low threshold, and accordingly there remains a need for improvement in electrode design which will provide the necessary stable electrode/heart wall interface, consistent with optimally reliable and stable thresholds, good sensing, and high pacing impedance.